This application claims priority to Japanese Patent Application No. P2000-221043
1. Field of the Invention
The present invention relates to position measurements for wireless communication, and, more particularly, to a wireless base station transmission timing position measurement system.
2. Description of the Background
A position measurement system using a wireless base station is disclosed in the Japanese Published Unexamined Patent Application No. Hei 7-181242. In that application, a terminal receives signals transmitted by a plurality of base stations, calculates the distance between each base station from which a signal is received and the terminal based on the timing delay properties of the received signal, and thereby measures the location of the terminal.
Many base stations are currently employed in a code division multiple access (CDMA) network or networks. In a typical CDMA network embodiment, a base station includes a system whereby the timing of transmission is determined in synchronization with the timing of the Global Positioning System (GPS). FIG. 2 illustrates the configuration of a base station system synchronized with the GPS. GPS artificial satellites 1, 2 and 3 exhibit an extremely accurate timing function, and each of the artificial satellites transmits a signal of a unique pattern. Upon receipt of a signal from a GPS satellite, wireless base stations 5, 6 and 7 have the capability to calculate the respective locations and time delays of a plurality of received waves, as well as orbit timing information, from the GPS satellites. The reception of this information from the GPS system enables each base station, like the GPS satellites, to maintain an extremely accurate time stamp. Through the use of this extremely accurate time stamp, each base station can transmit signals of a pattern unique to that base station at an accurately synchronized timing with other base stations in the network utilizing the GPS system for timing. Based on the same principle as discussed hereinabove with respect to the plurality of base stations, mobile terminal 4 has the capability to accurately measure time, and to thereby measure location.
Each wireless base station illustrated in FIG. 2 preferably includes an antenna having an unobstructed xe2x80x9cviewxe2x80x9d of at least a portion of a wireless communication field. Such an unobstructed view may, for example, be gained through the placement of the antenna on the roof of a building. Typically, the remainder of the base station apparatus is placed within the building. Signals transmitted from the GPS satellites 1, 2 and 3 are received by a GPS antenna 13 located on or near the base station, and the GPS antenna 13 passes to a GPS receiver 14 the estimated spatial position and time.
A reference clock generator 15 then calibrates the reference clock generator time, i.e. the reference clock generator oscillator, on the basis of the time information estimated from the GPS receiver 14, and thereby achieves an adjustment to the correct transmission time, according to the GPS time, for the attached base station 6. The reference clock generated by the reference clock generator 15 is supplied to a base band unit 12, and an RF unit 11. The base band unit 12 generates the signals to be transmitted from the antenna 10, and the RF unit 11 converts the signals to be transmitted into signals of a radio frequency for transmission via antenna 10.
In order to allow synchronization and signal reception, as well as position measurement, such as in a CDMA system, the transmission timing at which an electric wave is actually transmitted from an antenna must be very accurate. However, it will be apparent to those skilled in the art that, because a GPS antenna and a base station antenna are placed at a distance from the respective base band apparatus, the electric wave takes time to travel over cables to reach from the antennae to the base band unit 12. Further, the lengths of the cables connecting the antennae and the corresponding base band unit 12 vary with the layout of each site of installation. Moreover, since a reference clock generator 15 and a base band unit 12 each include filters for wave manipulation, the respective filtering systems can cause additional wave propagation delays. Additionally, as newly installed base station technologies may use different components, delays attributable to filters may differ or increase with each new generation of technology.
The time difference due to cabling and filtering at each base station is hereinafter referred to as a xe2x80x9cbase station transmission timing offsetxe2x80x9d or xe2x80x9coffsetxe2x80x9d. It will be apparent to those skilled in the art that transmission timing information is sent by a base station to allow a receiver to calculate propagation time, i.e. reception time at the terminal less transmission time, and thereby to allow for terminal location via triangulation, for example. However, due to the transmission timing offset, which is itself due to differences in cabling and filtering that cause the base station transmission timing offset to be intrinsic to each base station, propagation times can often not be properly calculated, because the transmission time received is not the correct transmission time, due to the offset. The timing at which a wave is actually transmitted from an antenna 10, versus the time at which the corresponding base band unit 12 intends to generate the wave, are different according to the cabling and filtering delays inherent in each system. In light of the transmission timing offset, a position measurement terminal notified of the time at which a base station intends to send a signal will deviate in the calculation of position the equivalent of the base station transmission timing offset.
A DGPS system calculates the position at a point of known location on the ground using GPS, and transmits the resultant compensation information by way of FM signals or the Internet. GPDS is often used with GPS-based systems. An embodiment of a GPDS system is described in Japanese Published Unexamined Patent Application No. Hei 9-311177. The use of GPDS allows for the compensation of delay lengths in propagation through the ionosphere, which delay lengths may vary from moment to moment, and according to the orbit of GPS satellites. However, the GPDS method is inapplicable to any position measurement system wherein the signal sources are cellular stations, because, in light of the fact that cellular base stations are at ground level, the intensity of signals transmitted from a nearby base station versus that from a distant base station may differ widely, thereby resulting in a relative distance problem, meaning that signals from many base stations cannot be measured at the same time at a single detecting station.
Therefore, the need exists for a system, apparatus, and method for compensating a base station for a transmission timing offset.
The present invention is directed to a transmission timing measurement apparatus for measuring timing of a signal transmitted from a wireless base station antenna. The transmission timing apparatus includes an accurate time reception antenna that receives an accurate time value, a cellular antenna that receives a reception time value of the signal transmitted from the wireless base station antenna, and a reference clock generator that receives the reception time value and the accurate time value. The reference clock generator compares the reception time value and the accurate time value into a reference output. The reception time value generally includes a transmission time stamp of the signal transmitted from the wireless base station antenna, and the reference clock generator preferably subtracts a propagation time from the accurate time value to form a first sum, and then compares the first sum to the transmission time stamp into the reference output. The reference output may adjust the timing of the signal transmitted from the wireless base station antenna, and one or more reference outputs may be stored in a memory unit.
The present invention is also directed to a transmission timing measurement system. The system includes an accurate time generator that transmits an accurate time value, at least one wireless base station antenna that transmits a first signal, and a transmission timing measurement apparatus having a first antenna and a second antenna that receives the accurate time value at the first antenna and the first signal at the second antenna, and that outputs an offset of the first signal from the accurate time value. The system may additionally include a center, and the offset for at least three of the wireless base station antennae may be stored in the center. The system also preferably includes a terminal. The terminal accesses the center and receives therefrom the offset for at least three wireless base station antennae in communication with the terminal. Either the terminal or the center may then perform a position location of the terminal.
The present invention is additionally directed to a method for measuring timing of a signal transmitted from a wireless base station antenna. The method includes the steps of receiving, at a first antenna, an accurate time, outputting, from the first antenna, the accurate time value at reception of the signal transmitted from the wireless base station, receiving, at a second antenna, the signal transmitted from the wireless base station antenna, outputting, from the second antenna, a reception time value of the signal transmitted from the wireless base station antenna, receiving, at a reference generator, the reception time value of the signal transmitted from the wireless base station antenna, and the accurate time value, and comparing, by the reference generator, of the reception time value and the accurate time value into a reference output. The method may additionally include the step of adjusting the timing of the signal transmitted from the wireless base station antenna according to the reference output. The method also preferably includes the steps of storing the reference output, accessing, by a terminal, of the stored reference output, receiving, by the terminal, of the reference output for at least three wireless base station antennae in communication with the terminal, and locating the physical position of the terminal according to the communication with the terminal from the wireless base station antennae and the reference outputs.
Thus, the present invention provides a system, apparatus, and method for compensating a base station for a transmission timing offset that do not require a site for a detecting station, as in Japanese Published Unexamined Patent Application No. Hei 9-311177, and that do not encounter multipath or relative distance difficulties.